The slowness with which the market for virtual-reality systems has grown is largely attributable to the high cost, large size and unwieldy nature of the head-mounted display (HMD) normally associated with such systems. These attributes of the HMD have restricted that market to specialty niches and have prevented its expansion into the juvenile and adult recreational area, where a vast market potential exists.
To achieve the depth perception that is of central importance in virtual reality system, typical HMDs use a pair of video displays, in conjunction with two separate and independent optical systems, one for each of the user's eyes. The two independent optical systems present right and left-hand images to the respective eyes that, together, simulate depth. The requisite image magnification and correction of color dispersion are typically addressed through an array of a half-dozen refractive lenses per image. Each of those arrays typically has a physical length on the order of double its effective focal length. The resultant HMD which includes two separate optical systems is both massive in physical size and weight, and wholly impractical for use in a recreational application, even if its price were affordable.
Although none of the prior art concerning displays touches on the teachings of the present invention, Johnson, in U.S. Pat. No. 5,161,057, teaches the use of small-scale diffraction grating facets molded onto large-scale Fresnel lens facets, to compensate color dispersion in solar-energy concentrators. While bearing superficial resemblance to certain aspects of the present invention, Johnson's teachings yield optical results that are inappropriate to display systems, primarily because their use would result in images that were color fringed or "haloed". Such halos may be of little consequence when focusing the solar spectrum onto a heat transfer device as in the device of Johnson, but the same halos would be an unacceptable, eye-straining flaw in a optical system for a display.
Johnson's approach to color correction imposes on the Fresnel lens a design constraint that its facet periodicities be consonant with the periodicities of the diffractive structure. This is necessary to ensure that the diffractive facet at the edge of each Fresnel facet is not truncated, an important consideration with a grating of such extremely low line density. A second, more serious problem results from the discontinuities in the diffractive structure caused by the Fresnel facets, as illustrated in FIG. 1A and the more detailed view in FIG. 1B. These discontinuities introduce the haloing problem observed and mentioned by Johnson (see Johnson col. 3, lines 19-22), though he offers no explanation for that puzzling phenomenon. A third drawback of Johnson's approach is that it requires concentricity of the Fresnel and grating structures, thus eliminating their off-axis displacement from one another as a convenient design option. An additional deficiency with Johnson's approach is that it is almost impossible to produce a phase continuity over the large step at the edge of the facets of the Fresnel lens (step height as much as 0.5 mm). Therefore, the coherent grating effect is limited to one Fresnel facet (approximately 40 grating periods). Such a grating causes collimated light to be spread 0.5-1.0 degrees. This is not significant in Johnson's solar energy application, but would cause significant aberrations in an imaging system such as the present invention.
FIGS. 1A and 1B, adapted from Johnson, illustrate Johnson's method of correcting color dispersion. Diffraction grating facets are formed as concentric rings on the Fresnel-lens surface. A segment of Fresnel lens 32 is shown in FIG. 1B, so that the facets, 37 of the color compensating diffraction grating can be seen more clearly. The color haloing problem previously discussed can be traced to the discontinuities introduced into the diffraction grating by Johnson's arrangement. The view of greatest enlargement shows the region at the edge of a single Fresnel facet. Note that while light ray A, that is parallel to the optical axis, passes through only one grating facet 37, light ray B, which typifies light rays that deviate from strict parallelism to the optical axis, passes not through one grating facet, but through two facets. In this example light ray B passes through grating facet 38 on the crest of the Fresnel facet, and then passes through a second grating facet 39 near the trough of the contiguous Fresnel facet.
Several prior art systems have made use of a single video display source, such as an LCD, but have used two different methods of splitting the image from the single display into separate right and left eye images. FIG. 2A illustrates a typical prior art method of beam splitting using "half mirrors". Optical system 200 of the prior art includes a display 205 and a projection lens 210 such that the image from display 205 is centered along optical axis 215. Disposed at 45.degree. with respect to axis 215 is a half mirror beam splitter 220. Half mirror 220 is constructed and arranged such that the reflection factor and transmission factor thereof are each about 50% with respect to the light from projection lens 210. Half mirror 220 splits the image from display 205 into two separate images 225 and 230. Full mirror 235 reflects split image 225 towards mirror 245 along an axis parallel to the other split image 230. Mirrors 240 and 245 serve to direct the two split images towards the user's two eyes.
FIG. 2B illustrates another typical prior art system 250 which incorporates an x-prism beam splitter, 270. System 250 uses a conventional display 255 and a projection lens 260 in order to generate an image along optical axis 265. An x-prism beam splitter 270 is disposed in this optical path in order to split the single image into separate right and left hand images for the user's two eyes. A conventional x-prism 270 is composed of four pieces of glass with a metal or dielectric coating on one side of each piece. These four pieces are bonded together to form an "X". Light entering cube 270 is split four ways, 25% each to the right and left and 25% each up and down. The light going up and down is entirely wasted. Conventional x-prisms employ the 50%-50% half mirror coatings described above, but since the light from lens 260 must pass through these coatings twice before exiting x-prism 270, the maximum theoretical efficiency is 25% and typical prisms are only 15% efficient. This low efficiency is due to the fact that if metal coatings are used for conventional x-prisms, there is considerable absorption, and for dielectric coatings, the efficiency is tempered by the need for a wide angular bandwidth.
As seen in the above two FIGS., 2A and 2B, the preferred method of the prior art for redirecting the split images towards a user's eyes is by the use of standard fold mirrors 240, 245 and 275, 280. The use of these fold mirrors places limitations on the permitted field of view of the HMD, increases the size of the HMD and the complexity of alignment of the optical elements.
It is an object of the present invention to incorporate a single display into a Head Mounted Display, while still providing separate right and left eye images. It is also an object of the invention to use low cost injection molded elements in the optical system. It is a further object to use Fresnel lenses and prisms in the optical system wherever possible. A further object of this invention is to provide a display that is software upgradable, so that the basic software will provide simulated depth perception, while the advanced software can provide true stereoscopic vision.